1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for recording and/or reproducing information with a laser beam, and more particularly to an optical information recording/reproducing apparatus provided with a beam rotation adjusting mechanism for precisely adjusting, in irradiating a track of an optical recording medium with plural laser beams for recording and/or reproduction, the positional relationship of the irradiating beams with respect to the track.
2. Related Background Art
Conventional information recording on a magneto-optical recording medium is achieved by moving a light beam emitted from a light source to a desired track on a disk (magneto-optical recording medium), then switching the light beam to an erasing power at a designated position and simultaneously applying an external bias magnetic field to the recording surface of the disk, thereby erasing the information recorded in the portion irradiated with the erasing power, and then switching the bias magnetic field to a direction opposite to that during erasure, and irradiating the above-mentioned erased portion with a light beam modulated according to a recording information signal, thereby recording the information on the recording surface of the disk. After the recording, the light beam is switched again to the reproducing power, and reproduction (verification) is conducted to check for errors in the data of thus recorded portion, and the recording operation of a cycle is completed upon confirmation of the coincidence of the recorded data and the reproduced data.
As explained in the foregoing, the information recording on the disk requires three operations of erasure/recording/reproduction, and there is required an even longer time in consideration of the time for awaiting the rotation of the disk in each operation. For this reason there has already been proposed overwriting technology, which can be generally classified into the magnetic field modulation method and the optical modulation method. In the former, a magnetic head for applying a bias magnetic field is positioned in facing relationship to the disk, and a light beam at the recording power irradiates the recording surface of the disk in a continuous or pulsed manner while the direction of the bias magnetic field applied by the magnetic head is modulated according to the information recording signal, whereby the erasure and recording of information are simultaneously conducted. This method, being capable of achieving erasure and recording of the information in one operation, can achieve the recording operation, even including the subsequent verifying operation, in a time equal to 2/3 of the time required in the foregoing method.
Also there is being developed a method of simultaneously irradiating a track of the disk with plural recording/reproducing laser beams from a laser array constituting the light source, and effecting the information recording with a recording beam while effecting the verification by a reproducing beam positioned therebehind on the track. This method enables further reduction in time, as all the recording operations (erasure, recording and reproduction) are conducted within one operation.
However, such a method employing the laser array requires new adjusting means for rotating the laser beams for precise positional adjustment, as the track has to be exactly irradiated with the laser beams.
In such optical information recording/reproducing apparatus, for irradiating the desired information track on the optical information recording medium with plural beam spots, there has been employed a configuration as shown in FIG. 1.
Referring to FIG. 1, a turntable 102 supports and rotates a disk-shaped optical information recording medium 101. An optical system, for irradiating the disk-shaped optical information recording medium 101 on the turntable 102 is composed of a fixed optical system, fixed to the apparatus, and a movable optical system including, for example, an objective lens 106 driven in the radial direction of the recording medium 101 (tracking direction of the beam spots) by an actuator (not shown) and a mirror 105 mounted on a carriage (not shown) driven in the radial direction of the recording medium for example by a linear motor (not shown).
In the above-mentioned fixed optical system 103, a two-beam semiconductor laser array 301 emits two laser beams, which are supplied, through a collimating lens 302 and a polarizing prism 303, to an image rotating element 104. The light image is rotated about the reference optical path by the image rotating element 104 and is directed to the mirror 105. In the above-mentioned movable optical system, the mirror 105 deflects the above-mentioned laser beams to a direction perpendicular to the recording surface of the recording medium 101, and the above-mentioned objective lens 106 condenses desired beam spots on the recording surface of the recording medium 101.
The reflected light from the recording medium 101 returns from the movable optical system to the fixed optical system, then guided through the image rotating element 104 to the polarizing prism 303, is further guided through the condenser lens 305 or through a 1/2-wavelength plate 304, another polarizing prism 303, the condenser lens 305 and a cylindrical lens 306, and is supplied to photosensors 307a, 307b, 307c. The above-mentioned image rotating element 4 is usually composed of an image rotating prism.
The photosensor 307a is used for obtaining information for the APC (auto power control) of the two-beam semiconductor laser array 301. One of the two beam spots on the recording medium 101 is used for servo control, such as tracking and focusing, of the beam spots relative to the information track, and the corresponding servo information is obtained from the photosensor 307b. A reproduced signal of the read information is obtained from the photosensor 307c.
The initial tracking adjustment of the beam spots on the recording surface of the recording medium 101 can be achieved by rotating the above-mentioned image rotating element 104 in a direction C. There are known two methods, as shown in FIGS. 2 and 3, in the beam spot formation on the information track. In a method shown in FIG. 2, two beam spots are formed on a same information track, and the information is recorded with a beam spot b, while the recorded information is read and directly verified by an immediately following beam spot a. In a method shown in FIG. 3, two beam spots irradiate different information tracks and simultaneously effect information recording and reading.
For obtaining a necessary and sufficient recording or reproducing ability, the two beam spots have to be aligned with the desired information track, with a precision of the order of a sub-micron. For this purpose, the rotating mechanism for the above-mentioned image rotating element 104 has to be highly precise and to have a high resolving power. FIG. 4 is a perspective view of a rotating mechanism for the image rotating prism conventionally constituting the image rotating element 104. The image rotating prism 104 adheres to a rotating seat 401, which is rotatably supported by a mounting plate 403 fixed to the frame of the aforementioned fixed optical system 103. The mounting plate 403 is provided with reducing gears 404 and a worm gear 406, which meshes with a gear portion 402 formed on the rotating seat 401. The rotation of the gear 405 by a wrench is reduced by the above-mentioned gears whereby the rotating seat 401 rotates and the beam spots effect a rotational movement by a small angle.
However, in such conventional configuration, since the initial fine tracking adjustment is conducted solely by the mechanism for rotating the image rotating element, the rotating mechanism is required to have a high precision and a high sensitivity for adjustment. For this reason there result drawbacks such as the complexity of the apparatus and a high cost, and the increased time required for adjustment. Also an added reflecting surface in the optical path of the optical system generates a phase difference, thus causing a deterioration of the signal reproducing performance.